As the name suggests, most electronic devices today are powered by the movement of electrons. But materials capable of efficiently conducting protons (the nucleus of the hydrogen atom) could be key to a number of important technologies to combat global climate change.
Most of the proton-conducting inorganic materials currently available require undesirably high temperatures to achieve sufficiently high conductivity. However, lower-temperature alternatives could enable a variety of technologies, such as more efficient and long-lasting fuel cells to produce clean electricity from hydrogen, electrolyzers to produce clean fuels such as hydrogen for transportation, solid-state proton batteries, and even new types of computing devices based on ionoelectronic effects.
To advance the development of proton conductors, MIT engineers have identified certain characteristics of materials that give rise to fast proton conduction. Using those characteristics quantitatively, the team identified a half-dozen new candidates that show promise as fast proton conductors. Simulations suggest that these candidates will perform much better than existing materials, although they still need to be fine-tuned experimentally. In addition to uncovering potential new materials, the research also provides a deeper understanding at the atomic level of how these materials work.
The new findings are described in the journal Energy and Environmental Sciencesin a paper by MIT professors Bilge Yildiz and Ju Li, postdocs Pjotrs Zguns and Konstantin Klyukin, and their collaborator Sossina Haile and her students at Northwestern University. Yildiz is the Breene M. Kerr Professor in the Departments of Nuclear Science and Engineering and Materials Science and Engineering.
“Proton conductors are needed in clean energy conversion applications, such as fuel cells, where we use hydrogen to produce electricity without carbon dioxide,” Yildiz explains. “We want to do this process efficiently, and therefore we need materials that can transport protons very quickly through these devices.”
Current methods of hydrogen production, such as steam methane reforming, emit a lot of carbon dioxide. “One way to get rid of it is to produce hydrogen electrochemically from water vapor, and for that you need very good proton conductors,” Yildiz says. The production of other important industrial chemicals and potential fuels, such as ammonia, can also be done using efficient electrochemical systems that require good proton conductors.
But most inorganic proton-conducting materials can only operate at temperatures between 200 and 600 degrees Celsius (about 450 to 1,100 degrees Fahrenheit), or even higher. Such temperatures require energy to maintain and can cause materials to degrade. “You don’t want to reach higher temperatures because that makes the whole system more complicated and the durability of the material becomes an issue,” Yildiz says. “There is no good room-temperature inorganic proton conductor.” Today, the only known room-temperature proton conductor is a polymeric material that is impractical for applications in computing devices because it cannot be easily shrunk to the nanoscale, he says.
To tackle the problem, the team first had to develop a basic, quantitative understanding of exactly how proton conduction works, taking a class of inorganic proton conductors, called solid acids. “You first have to understand what governs proton conduction in these inorganic compounds,” he says. By looking at the atomic configurations of the materials, the researchers identified a couple of features that directly relate to the materials’ proton transport potential.
As Yildiz explains, proton conduction first involves a proton “jumping from a donor oxygen atom to an acceptor oxygen. And then the environment has to rearrange itself and take the accepted proton away, so that it can jump to another neighboring acceptor, allowing for long-distance proton diffusion.” This process happens in many inorganic solids, he says. Figuring out how that last part works — how the atomic lattice rearranges itself to take the accepted proton away from the original donor atom — was a key part of this research, he says.
Researchers used computer simulations to study a class of materials called solid acids that become good proton conductors above 200 degrees Celsius. This class of materials has a substructure called a sublattice of polyanionic clusters, and these clusters have to rotate and knock the proton out of its original site so that it can be transferred to other sites. The researchers were able to identify the phonons that contribute to the flexibility of this sublattice, which is essential for proton conduction. They then used this information to screen extensive databases of theoretically and experimentally possible compounds, searching for better proton-conducting materials.
As a result, they found solid acidic compounds that are promising proton conductors and that have been developed and produced for a variety of different applications, but never before studied as proton conductors; these compounds turned out to have the right lattice flexibility characteristics. The team then ran computer simulations of how the specific materials they identified in their initial screening would behave at relevant temperatures, to confirm their suitability as proton conductors for fuel cells or other uses. Sure enough, they found six promising materials, with predicted proton conduction rates faster than the best existing solid acidic proton conductors.
“These simulations have their uncertainties,” Yildiz cautions. “I don’t want to say exactly how much higher the conductivity will be, but they look very promising. Hopefully this will motivate the experimental field to try to synthesize them in different ways and make use of these compounds as proton conductors.”
Translating these theoretical findings into practical devices could take several years, she says. The first likely applications would be in electrochemical cells to produce fuels and chemical feedstocks such as hydrogen and ammonia.
The work was supported by the U.S. Department of Energy, the Wallenberg Foundation and the U.S. National Science Foundation.
